The present invention relates to a new system for curing phenolic resins, and in particular to a system for using phenol-formaldehyde resols.
The curing of phenolic resols by the addition of strong acids is known. For example, hydrochloric acid, sulphuric acid, phosphoric acid, trichlorolacetic acid, and sulphonic acids are employed as acids, either alone or as mixtures. Most frequently, these acids are used in the form of aqueous solutions at concentrations ranging between 20 and 70%. (Such curing agents are also referred to herein as hardening agents or catalysts.)
For some applications, the phenolic resins are conventionally cured in the presence of reinforcing agents, e.g. cellulose or glass fibers, non-woven fibers made of high molecular weight polester or of polyvinyl chloride for example, or glass mat, or cloth made, for example, of an aromatic polyamide, glass or asbestos. After preliminary curing of the phenolic resin, the composite obtained is then finally hardened at a temperature generally in the region of 90.degree. C.
In the manufacture of some reinforced compositions it is sometimes essential to be able to increase the pot or shelf life of the phenolic resin at ambient temperature, on the one hand, but also to permit a high rate of hardening of the resin when the composition is cured at higher temperatures. It can be seen therefore that in practice there are two opposing problems to be solved. On the one hand, the resin must not harden too quickly in the cold but, on the other hand, it is also necessary that it retains a hardening potential which is sufficient to enable it to be quickly cross-linked when heated.
As examples of reinforced compositions where it is necessary to solve the problems outlined above, mention can be made of the manufacture of phenolic prepegs, the manufacture of phenolic materials prepared by filament winding, and the manufacture of articles by the "pultrusion" technique, a technique consisting of pulling the finished product by making it pass through a die.
The equipment employed for the manufacture of materials prepared by filament winding comprises, in brief, an impregnation trough containing the phenolic resin and curing agent, in which the glass fibers are impregnated. After impregnation the fibers are drained and then reeled around a mandrel before being hardened in an oven at temperature of between 60.degree. and 90.degree. C. The techniques employing pultrusion involve the impregnation of reinforcing agents with the aid of resins. This impregnation is carried out with impregnation troughs which contain the resin to which a curing agent has been added. The impregnation is followed by a heating stage which make it possible to carry out the polymerization of the resin and then a stage of drawing with the aid of a heated die in which the polymerization of the resin is continued. If appropriate, the heating stage may be carried out after the passage through the die. The reinforcing agents employed in the pultrusion technique generally consist of glass fiber rovings. The phenolic resins are particularly suitable for the use of the pultrusion technique. Whether the materials are manufactured by the filament winding techniques or by the pultrusion technique, it is essential, in order to avoid losing large quantities of resins that the phenolic resin does not harden too quickly in the impregnation troughs which are kept at ambient temperature. Furthermore, it is also necessary and advantageous that after impregnation of the fibers, the phenolic resin can harden when heated and that it does so quickly in order that the material obtained occupies the oven for a minimum time.
To solve these problems, it is possible to reduce the concentration of the acid hardener solution and to employ aqueous solutions which have an acid concentration below 20%. In this case, large quantities of water are carried along, which effectively increase the pot life at ambient temperature but which have the disadvantages of reducing the rate of hardening when heated and imparting imperfections to the finished products, in the form of blisters and micropits. It has also been proposed to replace the water completely with organic solvents to obtain dilute acid solutions. In this case, an increase in the pot life at ambient temperature is, in fact, observed, but so is a reduction in the rate of hardening when heated; moreover, it is observed that excessive quantities of solvents are found in the finished products and cause, in particular, a decrease in the mechanical properties of these products, which in most cases is reflected in cracks. In addition, the use of these solvents presents storage and toxicity problems which can only be solved by the use of costly extraction equipment. From an economic point of view, there is, therefore, no advantage in using curing agents dissolved in organic solvents.